1. Field of the Invention
The invention relates to a circuit, and more particularly to an ion sensing circuit.
2. Description of the Related Art
An ion-sensitive field-effect transistor (abbreviated as ISFET) is made by removing metal from the gate of an ordinary Metal-Oxide-Semiconductor Field-Effect Transistor (abbreviated as MOSFET) to be exposed to a solution. When a reference voltage used as a direct current bias voltage is placed into the solution to cause an electrolytic change of the solution, the channel current of the ISFET varies according to the ion (such as H+) concentration in the solution. Because the ISFET includes electrodes and the field-effect transistor, the ISFET can be incorporated with other electronic components to fabricate an ion sensing circuit that may be applied to biological medicine, analysis of water quality, and pH detection of unknown solutions.
As shown in FIGS. 1 and 2, each of a first and second conventional ion sensing circuit adjusts a gate potential of an ion-sensitive field-effect transistor 100, 200 using an operational amplifier 103, 202 to perform ion detection. An analog filter and an analog-to-digital converter are then used to filter and convert the electric potential of the gate into a digital signal.
Both of the foregoing conventional ion sensing circuits require an extra analog filter and an extra analog-to-digital converter for filtering and converting voltage signals from the ion-sensitive field-effect transistors into digital signals. Accordingly, both conventional ion sensing circuits may incur added design costs and increased circuit assembly complexity. In addition, for purposes of reducing noise, the conventional ion sensing circuits may include additional increased design complexity with respect to the operational amplifiers.